The electrical stimulation of various biological systems is known to the prior art. For example, pain suppression by electrical nerve stimulation is an accepted technique. Motor control through selective nerve and/or muscle stimulation has also been usefully employed. More recent developments include electrical treatment of spinal curvature and brain stimulation for various purposes including thalamic stimulation for pain suppression and cerebellum stimulation for the treatment of epilepsy and for motor control.
In each of the above mentioned electrical stimulation applications, the value of alternative electrode configurations is readily apparent. For example, in many direct nerve stimulation applications the nerves to be stimulated are sheathed with other nerve bundles, or bundle branches. Thus, to optimize the desired stimulation of a specific nerve bundle it is necessary to provide a specific orientation of the electrode or electrodes relative to the several nerve bundles. It has also been established that an electrode may migrate, either longitudinally or by rotating around the nerve bundles, thus requiring a reorientation of the electrodes relative to the nerve.
Within other environments, the optimization of the electrode configuration may be impossible, or nearly so, for some time after placement of the electrodes. For example, the effects of cerebellum stimulation are often delayed for several weeks after the implant. In this situation, it is impossible, at the time of implant, to accomplish anything more than the positioning of a plurality of electrodes with the optimization of the electrodes through which the stimulation is to occur being delayed. Thus, a system by which electrodes can be positioned, and later selected, is desirable for these and many other stimulation applications. In other instances, it may be desirable to stimulate various biological systems with a single implanted receiver by sequentially selecting differing electrode configurations. For example, a spinal curvature having two treatable curves may be treated through the implantation of a single stimulator with the output of that stimulator being sequentially alternated between the electrodes to treat each of the curves. Other multiple treatment situations may similarly be accomplished with a stimulator having the ability to sequentially select differing electrode configurations.
In addition to the above, it is also known that changing the location of the sites at which the stimulation energy is applied can reduce the fatigue, nerve fatigue, for example, commonly attending a repetitive stimulation at the same site or sites.
Electrodes having multiple leads and electrode contacts have been developed for altering the electrode configuration, an example being illustrated in U.S. Pat. No. 3,738,368, issued June 12, 1973. With this electrode, selective nerve bundle stimulation is accomplished by coupling less than all of the leads to a source of stimulation energy. The lead selection may be altered until the optimal combination is found. Within the system disclosed in the referenced patent, it is contemplated that the electrode will be positioned on the nerve and the leads brought out through the skin for temporary connection to a suitable source of stimulation energy. After several days of stimulation utilizing different combinations of leads, the most effective electrode configuration is determined and the unnecessary leads are severed.
Other systems have been proposed in which alternate output paths may be selected through the application of an external signal. For example, in U.S. Pat. No. 3,311,111 the use of bistable magnetic reed switches is proposed for the control of several stimulator functions including, pulse rate, voltage, current or duration as well as the selection of alternative output paths or leads. Within this system, a magnetic field is applied to effect the desired alteration with that alteration remaining effective until the reapplication of the magnet field. Of course, the system is susceptible to a "switching" by an extraneous magnetic field.
Although system in which the output path is selected in accordance with an external signal is disclosed in application Ser. No. 422,896, filed Dec. 7, 1973, now U.S. Pat. No. 3,888,261 which is commonly owned with the present application and which is hereby incorporated by reference. Within this system, alternating stimulus and control signals are transmitted to an implanted receiver with the receiver switching the stimulus signals between the outputs under the control of the control signals. The outputs each include independent channels with the receiver switching from channel to channel on the occurrence of a control signal.